Lubricating oil thickened to a grease with a mixture of silica and pyrogenic alumina



LUBRICATING OIL THICKENED TO A GREASE WITH A MIXTURE F SILICA AND PYROGENIC ALUMINA Peter B. Marsden, 31 Hudson St.,

Cambridge, Mass, and Gregor Berstein, 6

Maple St., Roxbury, Mass. No Drawing. Filed May 29, 1957, Ser. No. 662,315 4 Claims. (Cl. 252-28) This invention relates generally to grease compositions, and has particular reference to a grease having excellent shear stability and improved resistance to decompcsjtIon by water or water vapor, particularly during and after exposure to elevated temperatures.

Lubricating greases for many purposes have been prepared from mineral base lubricating oils by adding thereto various thickening agents, for example, metal soaps of fatty acids, to set up the oil into a grease-like consIstency. Such greases have adequate shear stability and viscosity retention, and a few have adequate resistance to decomposition by water, at operating temperatures below the melting or decomposition temperature of the organic thickening agent, but above this temperature, the additive loses its gelling power, permitting the grease to revert to the liquid form.

More recently lubricating oils have been thickened into greases by gelling the oil with certain colloidal metal or metalloid oxides formed from a hydrogel, since the greases so formed are satisfactory for use at much higher temperatures than those thickened with metal soaps. Examples of colloidal oxides commonly used for this purpose are silica, alumina, magnesia, zinc oxide, vanadium oxide, iron oxide, and mixtures thereof. The colloids for this purpose are prepared by first forming a hydrogel in the usual manner, for example, in the case of silica, by adding an acid to a sodium silicate solution. The hydrogel is then subjected to a somewhat complicated and expensive water-removing process, leaving the oxide in the form of an aerogel, which is thereafter incorporated into the lubricating oil. One method by which this is accomplished is by displacing the water with a volatile water-miscible organic solvent followed by evaporation, or by replacing the solvent directly with the vehicle from which the grease is to be formed.

Greases formed by the combination of such hydrogenic colloidal oxide gelling agents with lubricating oil are capable of use at elevated temperatures up to the thermal decomposition temperature of the oil without substantial loss of viscosity. However, it has been found that such greases have extremely poor resistance to decomposition by water or water vapor, particularly at elevated temperatures. For example, a grease gelled with colloidal silica, an oxide which has a relatively high thickening power, will disintegrate in boiling water in a matter of seconds.

Many additives have been proposed for increasing the "water resistance of greases of this type. However, the

only effective additives known are organic compositions such as organic acids, metallic soaps of organic acids, and amines. Such additives, at higher temperatures, lose their ability to inhibit disintegration of the grease by water or water vapor, probably as a result of partial or complete decomposition of the additive. Hence such greases are unsuitable for use at elevated temperatures in the presence of water vapor.

Such greases may also be unsuited for use in many other high temperature applications even if no water vapor is present at the elevated temperature, since once exposed to a temperature high enough to cause decomposition of theadditive, the water resistant quality is destroyed, and is not regained when the grease returns toa lower temperature. Hence the grease may thereafter disintegrate in the presence of water at a relatively low operating temperature. Additives of this type also have the disadvantage of requiring the use of more gelling agent to obtain the desired grease consistency, thereby increasing the cost of the product.

The principal object of this invention is to provide a grease composition of good shear stability which retains both viscosity and resistance to decomposition by water during and after exposure to elevated temperatures.

A further object of the invention is to provide a gelling agent for the manufacture of grease from lubricating oil which is formed of solely inorganic materials and which also imparts water resistance to the grease.

A further object of the invention is to provide an inorganic oxide thickened water resistant grease which is easier to manufacture and less expensive than previously known greases thickened with inorganic oxides.

Other objects of the invention will be apparent to those skilled in the art from the following description of a specific embodiment thereof.

- In accordance with this invention, it has been found that blends of pyrogenically formed alumina and one or more other metal oxides are capable of gelling lubricating oil into a grease which not only can withstand high temperature operating conditions without loss of viscosity, but which can also resist disintegration by water or water vapor during and after exposure to temperatures as high as the thermal decomposition temperature of the oil.

For the purpose of this application, a pyrogenic alumina is one which has been formed by a process of flame hydrolysis or oxidation, in which a finely dispersed aluminum salt, for example, aluminum chloride, is reacted at high temperature with the products of combustion of hydrogen or a hydrogen-containing compound such as natural or manufactured gas or fuel oil to produce aluminum oxide. The particles produced are of extremely fine particle size being in the range of from about 10 to about 40 millimicrons, and are collected from the gases of combustion in the dry form. The method of manu facture of such pyrogenic alumina does not form a part of the present invention, but is disclosed in an ap lication filed January 23, 1949, by C. A. Stokes and G. B. Kistiakowsky, Serial 129,089.

In a specific embodiment of the invention, parts by weight of a high viscosity index paraffinic petroleum oil were mixed with 7 parts by weight of an oxide blend consisting of 60% by weight of colloidal pyrogenic alumina having a particle size of between 10 and 40 millimicrons and 40% by weight of colloidal silica having a particle size of between 10 and 20 millimicrons. The oxides were first blended by mixing the materials in the dry state, and the blend was then thoroughly mixed with the lubricating oil to form a semi-fluid grease.

To determine the shear stability of the grease, it was tested by ASTM procedures, in which the penetration of a cone into the grease was first measured after the grease had been subjected to 60 strokes in a standard ASTM grease worker, and again after being subjected to 5000 strokes in the grease Worker.

For comparison purposes, a grease formed by adding 7 parts of silica alone to 100 parts of the above mentioned oil was also tested by the same procedure, with the following results:

The shear stability as determined by this procedure, is a measure of the ability of the grease to retain its viscosity in service and the above tests show that a grease thickened with the silica-alumina blend has over twice the shear stability of the grease thickened with silica alone.

To test the comparative water resistance of the above two greases, samples of each were immersed in boling water, and the time for disintegration noted. The grease thickened with silica alone disintegrated completely in boiling water in less than 3 seconds whereas the grease thickened with the silica-alumina blend was not d.sintegrated after 50 minutes in boiling water.

To test the retention of water resistance after exposure to high temperature of the grease formed with the oxide blend, a sample thereof was heated to 300 F. and maintained at this temperature for 24 hours. It was then cooled to room temperature and again tested in boiling water. It was found that the water resistance was not impaired, since the grease was not disintegrated after 50 minutes in boiling water.

Although a blend of 40% silica with 60% pyrogenic alumina has been found to have optimum properties, in regard to shear stability and water resistance, silica-alumina blends of other proportions have been found satisfactory for some purposes. For example, a grease thickened with a blend of 20% alumina and 80% silica when tested by the above described procedures, was found to have only slightly more shear stability than the grease thickened with silica alone, with an increase of penetration after 5000 strokes of 21%. However, its Water resistance was nearly as good as that of the 40% silica- 60% alumina blend, in that it resisted decomposition by boiling water for 40 minutes.

On the other hand, a grease thickened with a blend of 80% alumina and 20% silica was found to have excellent shear stability, showing an increase in penetration after 5000 strokes of only 12%, and a water resistance nearly as good as the 60% alumina, 40% silica blend, in that it resisted disintegration by boiling water for 45 minutes.

Hence it appears that the shear stability of greases thickened by silica-pyrogenic alumina blends increases with increasing percentages of alumina to a maximum at about 60% alumina, then decreases slightly with higher percentages of alumina. Also, the resistance of such greases to boiling water evidently increases greatly between alumina and alumina, then increases gradually to a maximum at 60% alumina, and decreases with percentages of alumina above 60%.

Although in each of the above disclosed examples, 7 parts of the silica-alumina blend were used for 100 parts of lubricating oil, this proportion is not critical, and was maintained constant merely to obtain comparative data. In actual manufacturing, suflicient thickener is added to the oil to bring the initial viscosity to a predetermined desired value.

Oxides other than silica may be used to blend with pyrogenic alumina to form water resistant high temperature greases. For example, magnesia, zinc oxide, vanadium oxide, iron oxide or mixtures thereof may be used instead of or in combination with silica. However, because of its superior thickening power, the silica is preferred. In particular, it is preferred to use a pyrogenie silica, which has been formed by the previously described flame hydrolysis process. Silica so formed has a considerably superior thickening power and is more easily incorporated into oil than previously used hydrogenic types of silica.

By the use of an oxide blend composed solely of pyro genie oxides, the incorporation of the blend into the lubricating oil is easily accomplished by a simple mixing process, as distinguished from the complicated methods necessary to incorporate the hydrogenic types of oxides into oil.

The oxide blends disclosed herein are suitable for use as thickening and waterproofing agents with both natural lubrication and synthetic oils. For example, instead of mineral mentioned in the above specific examples, synthetic oils such as polyglycols, silicone oils, and high molecular weight aliphatic diesters may be used with equally good results.

Since certain other obvious modifications may be made without departing from the scope of the invention, it is intended that all matter contained herein be interpreted in an illustrative and not in a limited sense.

We claim:

1. A grease composition, consisting essentially of a lubricating oil and a thickening additive composed of colloidal oxides incorporated therewith in an amount sufficient to gel the oil to a greaselike consistency, said additive consisting essentially of from about 20% to by weight of colloidal silica, with the remainder being colloidal pyrogenic alumina formed by the fiame hydrolysis of an aluminum halide.

2. A grease composition consisting essentially of a lubricating oil and an oxide mixture incorporated therein in an amount to gel the oil to a greaselike consistency, said oxide mixture consisting essentially of silica having a particle size of between 10 and 40 millimicrons and from 20% to 80% by weight pyrogenic alumina formed by the flame hydrolysis of an aluminum halide and having a particle size of between 10 and 40 millimicrons.

3. A grease composition consisting essentially of a lubricating oil and a thickening agent in an amount sufficient to gel the oil into a greaselike consistency, said thickening agent consisting essentially of a mixture of about 40% by weight of a finely-divided silica and about 60% by weight of pyrogenic alumina formed by the flame hydrolysis of an aluminum halide.

4. A grease composition consisting essentially of a lubricating oil selected from the group consisting of mineral oils, polyglycols, silicone oils, and high molecular weight aliphatic diesters and a thickening agent consisting essentially of from about 20% to 80% by weight colloidal silica and the remainder pyrogenic alumina formed by flame hydrolysis of an aluminum halide, said thickening agent being present in an amount sufl'icient to gel said oil to a greaselike consistency.

References Cited in the file of this patent UNITED STATES PATENTS ber Filler, by C. G. Love et al., April 24, 1946, Oflice of Military Gov, for Ger. (U.S.), pages 1-15. 

1. A GREASE COMPOSITION, CONSISTING ESSENTIALLY OF A LUBRICATING OIL AND A THICKENING ADDITIVE COMPOSED OF COLLOIDAL OXIDES INCORPORATED THEREWITH IN AN AMOUNT SUFFICIENT TO GEL THE OIL TO A GREASELIKE CONSISTENCY, SAID ADDITIVE CONSISTING ESSENTIALLY OF FROM ABOUT 20% TO 80% BY WEIGHT OF COLLOIDAL SILICA, WITH THE REMAINDER BEING COLLOIDAL PYROGENIC ALUMINA FORMED BY THE FLAME HYDROLYSIS OF AN ALUMINUM HALIDE. 